The present disclosure relates in general, to the field of carbon monoxide (CO) boilers. More particularly, the present disclosure is directed to a water cooled CO boiler floor with screen gas distribution inlet.
CO boilers are installed in the exhaust gas stream of fluid catalytic cracking units, which are comprised of a reactor and a regenerator. CO boilers are integral parts of the fluid cracking units, but they may be arranged so that the CO boiler could be operated independently, or taken out of service, without affecting the operation of the cracking unit.
Finely divided catalyst suspended in the gaseous vapors flows continuously in a cycle from the reactor to the regenerator and back to the reactor in fluid catalytic cracking units. Gas oil feed stock is injected into the hot regenerated catalyst line just before it enters the reactor. Hydrocarbon vapors leave the reactor through cyclone separators, which return the entrained catalyst to the reactor bed, and the cracked petroleum products are separated in the fractionator.
In the reactor, the catalyst accumulates a carbonaceous deposit. The spent, or carbon coated catalyst, is transported to the regenerator by injecting compressed air into the catalyst stream. Additional air is supplied to the regenerator directly to burn the carbon from the catalyst. The heat of combustion is absorbed by the regenerator catalyst, which, in turn, heats the oil feed stock to effect vaporization. The oil vapors and catalyst are then discharged into the reactor to begin the cracking and refining process.
The CO gases are discharged from the regenerator through cyclone separators, to remove as much of the entrained catalyst as possible before they enter the CO boiler for heat recovery prior to their discharge to the atmosphere. However, catalyst particles may remain mixed with the CO gases. The problem with these catalyst particles is that they are abrasive and can erode and damage the tubes as these CO gases and entrained catalyst pass across the tubes.
The CO boiler was developed to recover the heat discharged from the catalytic regenerator. Please refer to FIGS. 1a and 1b. FIG. 1a illustrates a side and plan view of a prior art elevation circular CO boiler. FIG. 1b illustrates a side view of another prior art top supported circular CO boiler with an integrated bustle. The combustible content of the gas stream is the result of the incomplete burning of the carbon at low temperature with, in most instances, a deficiency of air. The unburned combustibles consist primarily of carbon monoxide with some traces of entrained hydrocarbons. In catalytic crackers, it is desirable to burn off the carbon to produce a maximum of CO instead of CO2 since a cubic foot of air combines with twice the amount of carbon when as CO is made.
CO boilers are especially designed to obtain complete burning of the combustibles in the CO gas stream. The primary furnace is the critical part of a CO boiler from a combustion point of view because this is where the CO gas, the supplementary fuel and combustion air must be thoroughly mixed and burned.
The furnaces, both secondary and primary, and the boiler tube bank are designed as a single integrated boiler unit supported at the top, with provision for downward expansion. As shown in FIG. 1b, the primary furnace is below the bustle and the secondary furnace is above the bustle.
The supplementary fuel burners, and the CO gas nozzles are arranged for tangential firing to make the gases swirl, thus thoroughly mixing them to promote rapid and complete burning. Since CO boilers are often located at refineries, the supplementary fuel is usually refinery gas. The fuel burners are arranged in a staggered pattern with respect to the CO gas nozzles. The wall tubes are covered with refractory to minimize radiant heat absorption, thus facilitating the burning of the CO gas with a minimum amount of supplementary fuel. The wall tubes also cool the refractory, thus protecting the refractory material when firing only supplementary fuel.
The CO gas and combustion air windboxes or distribution chambers are designed as an integral part of the furnace. This provides a simple water cooled arrangement for the high temperature CO gases and eliminates difficult and expensive differential expansion and seal problems.
The secondary furnace, located immediately above the primary furnace, provides extra space for completing the combustion of the fuel and for radiant heat absorption. The economizer for preheating the boiler feedwater is located above the boiler, thus occupying a minimum of ground space.
A superheater is used to raise the steam temperature beyond the saturation point by transferring heat from the hot gases to the steam conveyed within the superheater tubes. An attemperator is used to regulate the steam temperature.
The CO gas plenum is a pressurised housing containing the CO gas at positive pressure and delivers the CO gas into the primary furnace. Forced-draft fans supply air for combustion.
To provide for the independent operation of the CO boiler without interfering with the operation of the regenerator, water seal tanks are installed so that the CO gases from the regenerator may be directed through the boiler or bypassed around the boiler directly to the stack.
Waste gas CO inlet ducts are typically arranged with adequate straight length for uniform gas distribution. The problem occurs when space and overall CO waste gas steam generator height and volume are limited, which may cause problems with adequate and effective incineration and steam generator performance.
Given the above, a need exists for a new and improved CO boiler, and in particular a CO boiler that provides adequate and effective incineration and steam generator performance in limited space while overcoming the problems associated with catalyst particles, which remains of significant commercial interest in the industry.